This invention relates generally to an improved resistance radiant tube heating element, more particularly to a heating element and method of making such element which has improved watt-density loading while maintaining a high power level.
Radiant tube heating elements generally operate in a tubular container which is mounted in a furnace. This protects the heating elements from being attacked by the gas atmosphere in the furnace. Within the tubular container is a length of metallic conductor or rod or resistance wire which heats due to resistance to flow of electric current. The length of rod or wire is ordinarily arranged in such a fashion that it is coiled or folded within the tubular container and connected to two terminals which usually extend from one end of the tubular container so that the two terminals can be attached to a power source.
Furnaces are usually designed with a multiplicity of heating elements to provide relatively uniform heating throughout the furnace. The heating elements preferably operate at a high kilowatt rating so that their physical size and the furnace size can be kept to a minimum; thus it is advantageous to provide elements that operate at a high-power.
Many furnaces are designed in such a way that the number of elements and tubes are limited; therefore, a high wattage is necessary to meet the heat requirements for the work passing through the furnace. One technique for generating designs for the elements involves reducing the diameter of the heating wire, which increases the wattage. However, this increases the watt density (watts per square inch of surface area radiating) of the wire which can cause or contribute to the premature failure of the heating wire or rod. A second design technique involves the installation of additional elements of the same size. However, this involves an increase of the physical dimensions of the furnace, which increases the heat losses and further increases the heating requirements and investment. Further, in many cases, the furnace is not equiped to be fitted with additional heating elements.
In some cases, the conductor is composed of graphite which is encased within the container and which contains an inert atmosphere such as nitrogen to prevent oxidation of the graphite. These elements are more expensive than metallic conductor elements and they are normally used in furnaces where the available space for the elements is limited. Graphite conductor elements can operate at a higher watt loading than metallic conductor elements and they are sometimes used for this purpose even though they require a non-oxidizing atmosphere for the element and water-cooled terminals to prevent overheating of the terminal area which adds to the costs.
In any type of resistance heating element the power is measured in watts (or kilowatts), wherein W=EI; and hence W=E.sup.2 /R where W=watts, I=current, R=resistance and E=volts. Thus, with these basic conventional electrical relationships, it is apparent that the heating capacity or power of any element may be increased either by increasing the voltage (E) or reducing the resistance (R) of the conductor, assuming that the other (E or R) remains constant. The voltage normally will be dictated by the design of the furnace and the heating elements must be designed to the assumed, or selected, or existing, voltage utilized by the furnace. Also, the power or wattage of the furnace and each element is predefined. Thus, if voltage and wattage are known, the resistance of the element is thereby defined.
The required electrical resistance is achieved by controlling three variables. First, selecting a suitable alloy such as a conventional nickel-chromium alloy or an iron base alloy (e.g. iron-chromium-aluminum alloys) which has a known electrical resistance. There are many commercially available nickel-chromium alloys, and iron based alloys designed for use as heating elements. Second selecting a particular size and shape (smaller conductors have greater resistance per unit length). Third, determining the length required to develop the total resistance required (longer conductors have greater resistance). When a potential solution is formulated using the three selection options above, it must be evaluated from several perspectives to see if it would be feasible to produce such an element. These perspectives include the dimensional limitations on the element (will it fit in the furnace), the spacing of the conductor loops in the element and watt loading that would result.
As indicated above, one of the critical variables that must be considered in designing heating elements is the watt-loading on the conductor in the element. Watt-loading, or watt-density, is defined as the watts.+-.surface area of the conductor. In fact the watt-loading, or watt-density, is essentially a limit on the heat that can be generated by a conductor of any given diameter before it will suffer physical damage. The maximum depends on several factors including the material of the wire and the temperature to which the furnace is heated. Expressed another way, if the watt-loading is too high, this will result in a significant premature failure potential of the element. Premature failure results when the rod or wire loses its physical integrity. The loss of physical integrity can be identified or determined by either the rod or wire becoming so hot that the interior of the wire becomes liquid which melts through wire, which in turn will result in loss of electrical continuity, or by the rod or wire bending or sagging in use to such an extent it will touch another portion of the wire, or the casing in which it is maintained which in turn will cause shorting. In either case, the required electrical continuity of the wire is lost. Hence, as used herein, safe watt loading or safe watt density means a watt loading or watt density which if exceeded will result in loss of physical integrity which in turn means that the wire will either melt, or in its designed setting will sag to such an extent a short will occur.
On the other hand, it is desirable to increase the wattage of each heater element so as to increase the amount of heating provided by the heating element, the heating being equivalent to the watts. One way to increase the watts without increasing the watt loading would be to increase the diameter and the length of the wire or rod. This may not be feasible, however, because the additional length and/or diameter adds volume to the heating element and there may not be ample or sufficient space within the available space within the container to contain this additional volume and as wire size increases, bending or forming the wire becomes much more difficult.
Another limitation in heating element design is the electrical resistance of the terminal. If the current required by the design is too high, it may be necessary to water-cool the terminals which is an added cost to the furnace operator.
Thus, in designing conventional electrical resistance wire heating elements for furnaces, a barrier is reached which imposes a limitation on the wattage of a given heating element utilizing a wire or rod of optimum size.